#include<iostream>
using namespace std;

const int N = 1110;
char str[N];

int main()
{
	cin >> str;
	int flag = 0;
	int i = 0;
	while (str[i])
	{
		int c = 0;
		int j = 0;
		while (str[j])
		{
			if (str[i] == str[j])
				c++;
			j++;
		}
		if (c == 1)
		{
			flag = 1;
			cout << str[i] << endl;
			break;
		}
		i++;
	}
	if (flag == 0)
		cout << "no" << endl;
	
	return 0;
}
